add storageLayout to solc output selection

buidler/task-get-compiler-input.js

+// adds storageLayout to solc outputSelection, necessary for storage gaps
+
+const { internalTask } = require('@nomiclabs/buidler/config');
+const { TASK_COMPILE_GET_COMPILER_INPUT } = require('@nomiclabs/buidler/builtin-tasks/task-names');
+
+internalTask(TASK_COMPILE_GET_COMPILER_INPUT, async (args, bre, runSuper) => {
+  const input = await runSuper();
+  input.settings.outputSelection['*']['*'].push('storageLayout');
+  return input;
+});